Electric furnace



Patented May l, 1945 Renzr'o' Voltcrra,Attlcboro, Mass., assignor to Metals & Controls Corporation, Attleboro,

"f-IMass.. afcornoration of Massachusetts Tl Applicatinnoctqberv, i943, seria1No.505,2c7

w.; l f2.6 Claims'. (Cl. 201-76) This vinvention,relates to `electric furnaces, and with regard to certain morespeciic features, to resistance-wound electric furnaces for high temperature heating. v y y Among the several objects oftheinvention may be noted theprovision of a resistance-wound furnace which employs a minimum amount of the usually-used expensive platinum but which does not require a vacuum'or hydrogen to prevent oxidation of a substitute for said platinum, and which therefore is cheaper and more safe to operate; the provision of the furnace of the class described which, because of the absence of hydrogen can safely be cooled immediately. in air, thus saving operating time; the provision of a furnace in the class described which requires less resistance wire for a given heat output than was heretofore used and which is therefore also less costly. Other objects will be in part obvious and in part pointed out hereinafter.

The invention accordingly comprises the elements and combinations of elements, features of construction, and arrangements of parts which will be exemplified in the structures hereinafter described, and the scope of the application of which will be indicated in the following claims.

In the accompanying drawing, in which is illustrated one of various possible embodiments of the invention, v

Fig. 1 is a longitudinal diagrammatic section of a typical inutile furnace embodying the invention;

Fig. 2 is an enlarged side elevation showing a detail of the resistance wire used in the furnace; and,

Fig. 3 isa vertical section taken on line 3 3 of Fig. Z.

Similar reference characters indicate corresponding parts throughout the several views of the drawing.

High-temperature resistance heating furnaces 'have heretofore been made with all-platinum wire, but this is so inordinately expensive, that the furnaces are impracticable for industrial use and are quite expensive even for laboratory use. Inorder to overcome this disadvantage, furnaces have been proposed which use molybdenum resistance wire, but this solution of the problem introduces the problem of preventing oxidation and vaporization of the molybdenum at the high tem- 5 peratures which prevail.

Thus in such molybdenum-wound furnaces, in order to prevent the molybdenum from oxidizing at the elevated temperatures it has been the practice to flow a continuous stream of hydrogen around it while hot. 'The use of hydrogen is expensive, and Very dangerous for if air leaks occur the adm ixture of oxygen 4with the hydrogen at high temperatures may result in an explosion. It is for this reason that molybdenum Wired furnaces have not come into general industrial use. They are almost exclusively confined to use in scientific laboratories.

Referring now more particularly to the drawing, numeral I shows a mullle furnace which consists of a shell 3 composed of a refractory or heat insulatingv material. In the chamber 'I of the furnace is the mullle 9 about which is placed the platinum-clad molybdenum wire I3. The wire is connected between terminals I5.

The wire I3 is composed of a relatively large molybdenum core 2I on the outside of .which is a relatively thin platinum sheath 23. Since the molybdenum furnishes the primary electrical resistance for converting electrical into heat energy its cross section is primarily determined by the electrothermal conversion requirements. Molybdenum has a higher resistance coefficient than platinum and since the furnace has as its primary function the conversion of electrical into heat energy, the furnace is cheaper because of the shorter length of wire required. The platinum thickness needs only to be enough to cover lthe molybednum so as to prevent Vaporization and oxidation of the molybdenum at the furnace temperatures desired. Its thickness may rangefrom several thousandths of an inch down to several ten thousandths.

If desired, the molybdenum core 2| may lbe made hollow as a tube, provided the ends and inside are covered up with platinum so that contact of air does not occur anywhere on the inner molybdenum. The tube may ,th/en constitute a muiile by itself. Other cross sections may also be used.

The sheath of platinum 23 may .be plated on the molybdenum core in order to obtain the thin coating desired, or any other suitable processes may be used for the purpose. f

It is to be understood that although a muifle type of furnace is shown, the invention is applicable to other furnaces. For example, resistance elements may be hung on the walls of the furnace without the interposition of any other material between the resistance and the articles to be heated.

It is to be understood that, although molybdenum is shown for the core of the wire, other materials may be'used which require protection against oxidation at high temperatures, which is protected at operating temperatures which are to be preferredto an all-platinum wire, such would bring about oxidation of the molybdenum. as for example tungsten. Molybdenum however 4. In an electric resistance heating furnace, ,is preferable. a resistance Wire comprising a. molybdenum core In view of theabove, it, will be-seen that the 5 of a cross ,section providingy forsubstantially ali several objects fix-.ifl the linver'ition are achieved and?l of thehefat transformation, de'siredinview of the other advantageous results attained. current value used, and a platinum sheath on v platinumfsheatlronsaid"cor lwherebysaid oi'e As many changes could be made in the aboveu .1. y"said core, whereby said core is protected at temconstructions without departing from the scope peraturesvwhich would bring about oxidation of of the invention, it is intended that alliniaiQtr;m lo tliemolybdenum said platinum sheath being of a few thousandths ofy an the accompanying drawing 'sha1 as illustrative and not in a limiting 'sens p I'claim:` n l l w prising a molybdenum core 1. In an electric resistancfhating'lfurnac 15 "fs`ubstaxtilffcrs'sf section, and a platinum a resistance wire comprising a molybdenumpcore sheath@ ereon of a thickness not over the order and a platinum sheath thereon. U offe'w tousandths of an inch. 4'2. fIn.arrelectric?resistance.fheatingzifurnacesa :s 16'.slnfanfelectricirsistancesheatingfurnacea resistance wirefcomprisingitfamolybdemnnzzcore@ res'istancewirecomprisingiametalrcoreiofacross-f and?azrelativelyithiniplatinumwsheath@ thereon-z 320 section'providingforsubstantially' fallrof't -3.iInanaelectricresistance'aheatingefurnace a transformation desired in view No istance heating furnace, a 

